Wastewater such as sewage streams generally have various naturally occurring and/or man-made contaminants, notably organic contaminants. In a remarkable display of the versatility of nature, some naturally occurring microorganisms have the ability to consume these contaminants for their own life processes, thereby turning what is an undesirable pollutant into (for their purposes) a beneficial nutrient or food source. The wastewater treatment industry frequently capitalizes on the ability of these microorganisms by using such microorganisms in facilities that treat wastewater streams to destroy the contaminants and break them down into basic compounds. Wastewater streams are fed into tanks or ponds that maintain conditions conducive to microorganism activity. Typically, the microorganisms which consume the targeted contaminants are mesophilic and thrive at temperatures in the range of about 25 to about 50 degrees Celsius.
The desired result of this type of wastewater treatment is the destruction of organic contaminants, but a by-product of this type of treatment is the production or increase of a biomass or biosolids comprised of the microorganisms. The biosolids yield from waste water treatment can range from about 0.1 pound of biosolids per pound of biological oxygen demand (BOD) removed to about 1 pound of bacteria per pound of BOD removed. A more typical range of biosolids yield is from about 0.3 pounds to about 0.6 pounds of bacteria per pound of BOD removed. Disposal of this biosolids may still be problematic, even after many contaminants have been consumed by microorganisms. One problem arises from the pathogenic nature of many microorganisms, such as the Fecal Coliform group of organisms; although such microorganisms have proven beneficial in consuming contaminants, they themselves may pose a danger to human health and are disease causing organisms. These include but are not limited to certain bacteria, protozoa, viruses and viable helminth ova. Regulations by states and/or the federal government impose restrictions upon land disposal of materials containing pathogenic microorganisms. It is desirable to treat biosolids so that one can easily and legally dispose of the biosolids on land or under ground. Suitably treated biosolids may even prove to have beneficial uses. Under certain circumstances, it may be used as a soil conditioner or fertilizer.
Another problem with the biosolids may arise from the sheer volume of biomass generated. Costs associated with the production and disposal of biosolids include both capital costs and operating expenses, such as biosolids disposal costs, trucking costs, material handling costs, management costs, and liability costs associated with disposal. Most if not all of these costs depend on the volume of biosolids at issue, and a reduction in the amount of biosolids can make an economically unfeasible operation into a profitable one. Methods which will reduce the mass and/or volume of biosolids to be disposed have significant commercial and environmental benefits.
Biosolids also contains other materials including microorganisms which are not pathogenic in nature. Typically the biosolids includes a group of microorganisms that thrive in what is generally referred to as the thermophilic temperature range. These thermophilic microorganisms are normally not harmful to humans, and there are both aerobic and anaerobic bacteria that thrive within the thermophilic range. This invention is especially interested in the aerobic microorganisms. Although the temperature ranges for classification of bacteria varies somewhat depending upon who is describing the range, thermophilic activity usually takes place within the range of from about 45.degree. C. to about 70.degree. C. In contrast, pathogenic bacteria usually thrive within what is referred to as a mesophilic range which is from about 25.degree. C. to about 37.degree. C. or the normal body temperature of humans, and may begin to die at about 38.degree. C.
Therefore, various methods have been proposed and practiced for treating the biosolids that results from treatment of wastewaters. Biosolids may be treated aerobically or anaerobically, with different microorganisms, conditions and results. Among the methods of biosolids treatment is autothermal thermophilic aerobic digestion ("ATAD"). ATAD capitalizes on the presence of materials in the biosolids such as naturally occurring microorganisms which are not pathogenic or harmful to humans but which will kill pathogenic microorganisms. Typically, these are thermophilic microorganisms which thrive at temperatures of from about 45.degree. C. to about 70.degree. C.
A preferred temperature for thermophilic microorganisms is approximately 65.degree. C. When this preferred temperature is maintained during the treatment of a wastewater biosolids, the reaction time for destruction of mesophilic microorganisms at 65.degree. centigrade for purposes of meeting governmental regulations is approximately three quarters of an hour, as established by the Environmental Protection Agency's Standards for Use and Disposal of Sewage Biosolids, 40 CFR, Part 503. Three hours is an easily obtained processing time for most biosolids treatment facilities, since biosolids is often pumped once every twenty four hours from the waste water treatment plant.
In a typical ATAD process, biosolids resulting from wastewater treatment is treated in a reactor, which operates at a temperature in the thermophilic range, i.e., from about 45.degree. C. to about 70.degree. C. Temperatures above the above this range do not allow the thermophilic microorganisms to thrive and may even result in their destruction. Within this temperature range, thermophilic microorganisms are active in an aerobic process where they consume oxygen, which must be provided in the solution.
An advantage of an aerobic process using thermophilic microorganisms is that their use of oxygen is an exothermic reaction. The heat released as a result of this reaction raises the temperature of the biosolids solution. As the temperature rises above the mesophilic range, mesophilic microorganisms are killed and consumed by thermophilic microorganisms. It has been estimated by others that 9000 BTUs may be released for every pound of volatile suspended solids destroyed. The interrelated cycle processes in which exothermic reactions trigger additional exothermic activity by thermophilic microorganisms results in an autothermal process and thereby creates an autothermal environment by virtue of the maintenance of relatively high temperatures.
Pathogens could also be destroyed through the direct application of heat from an outside heat source to the biosolids solution. By directly heating the biosolids to temperatures that are inhospitable for mesophilic microorganisms, these pathogens may be killed. However, this type of treatment (without the action of thermophilic microorganisms) is costly and wastes energy, since the amount of heat that must be directly applied to raise the temperature of the biosolids mass is substantial.
A major challenge in operating an aerobic biosolids treatment process is to keep the process sufficiently aerobic by meeting or exceeding the oxygen demand while operating at the elevated temperatures in which thermophilic bacteria thrive. One reason why this is difficult is that as the process temperature increases, the saturation value of the residual dissolved oxygen decreases. That is, a higher temperature results in less oxygen remaining in the biosolids solution. Another reason is that the activity of thermophilic microorganisms increases with higher temperature. This higher activity results in increased oxygen consumption by the microorganisms. Hence, greater amounts of oxygen must be imparted to the biosolids solution.
Another major challenge is to operate the process in an autothermal condition while still maintaining some control over the operating temperature. In an autothermal process, the process operates at a temperature higher than ambient without adding heat or by adding less heat than would ordinarily be needed to maintain the process at that temperature. In the biosolids treatment industry, autothermal processes capitalize on the exothermic nature of the action of the thermophilic bacteria in breaking down and consuming mesophilic bacteria or other organic compounds. The use of autothermal processes can obviate the need for external heat supply. However, it is still desirable or necessary to have some means of controlling the temperature of the process.
The need to control temperature has been previously identified and discussed in U.S. Pat. No. 5,587,081, which discloses a method of controlling temperature by varying the proportion of fresh air versus recycled air injected into the biosolids. By increasing the amount of fresh cool air introduced, the reactor is cooled. However the inventor believes it is important to use fresh air in the injection process because recycled air is not as effective in providing oxygen for thermophilic bacteria to thrive. The process described in U.S. Pat. No. 5,587,081 does not appear to take into account the fact that recycled air, although warmer than fresh air, has less oxygen and will generate less exothermic reaction and heat from the thermophilic microorganisms. The recycled air has a lower content of oxygen than is found in ambient air. This results in less oxygen being imparted to the biosolids solution by the recycled air. Although at first glance, it may appear that the effect of the reduced oxygen content is minimal because the reduction in oxygen may be only a few percent, in practice the reduced oxygen content results in insufficient oxygen being imparted to the solution to create a truly aerobic environment for the aerobic microorganisms to thrive.
Various apparatus and methods have been used to inject an oxygen containing gas stream into a biosolids solution. For example, spargers, diffusers and aerators of various designs and configurations have been used. It is the inventor's opinion that the best apparatus to deliver the necessary oxygen is the aeration jet. One such aeration jet has been developed by Mass Transfer Systems, Inc., ("MTS") 100 Waldron Road, Fall River, Mass. A product brochure by MTS is enclosed herein and incorporated by reference. By using the aeration jet, it is possible to create finer air bubbles along with higher shear which results in greater introduction of oxygen into the biosolids solution. There are many other advantages associated with the aeration jet, including better mixing. As the biosolids treatment occurs and mesophilic bacteria are broken down, carbon dioxide, water and ammonia (as well as other organic compounds) are produced when the protoplasm within the cell is released into the biosolids solution. The ammonia raises the pH of the solution and causes a noxious odor. Additionally, cell breakdown results in foam. It is desirable to have some means to treat odor and foam.